feat: update database schema for user tracking

src/db.ts

@@ -11,7 +11,9 @@ export function initializeDatabase() {
       due_date TIMESTAMP NULL,
       completed BOOLEAN DEFAULT FALSE,
       completed_at TIMESTAMP NULL,
-      group_id TEXT NOT NULL
+      group_id TEXT NULL,
+      created_by TEXT NOT NULL,
+      FOREIGN KEY (created_by) REFERENCES users(id)
     );
     
     CREATE TABLE IF NOT EXISTS task_assignments (
@@ -24,10 +26,9 @@ export function initializeDatabase() {
     );
     
     CREATE TABLE IF NOT EXISTS users (
-      phone_number TEXT PRIMARY KEY,
-      wa_id TEXT NOT NULL, -- WhatsApp's @s.whatsapp.net ID
-      name TEXT,
-      last_seen TIMESTAMP
+      id TEXT PRIMARY KEY, -- WhatsApp user ID (normalized phone number without +)
+      first_seen TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+      last_seen TIMESTAMP DEFAULT CURRENT_TIMESTAMP
     );
     
     CREATE TABLE IF NOT EXISTS groups (